Electrochromic devices take advantage of materials that are able to change their optical properties in a reversible and repeatable way under the application of an electrical current or potential. These materials include inorganic semiconductors and certain conjugated polymers.
These devices have the advantage of being able to tune through various redox states by the application of a small voltage to the devices as opposed to the dependence of external lighting conditions by photochromic materials. Known configurations of electrochromic devices include solution phase electrochromics where the diffusion of the electrochromic species from the solution to the electrode is followed by a redox reaction on the electrode surface accompanied by a color transition. Liquid crystals take advantage of alignment induced by an electric field, which induces a change in optical density of the material. A drawback of liquid crystals is that most require a constant supply of power to generate the effect. Another configuration includes Suspended Particle Devices (SPDs) where millions of suspended particulates are sandwiched between two transparent electrodes. Application of a potential leads to alignment of the particulates resulting in the transmission of light. The intensity of transmitted light can be controlled by varying the applied potential. Still yet another configuration includes electrochromic films where thin films of organic or inorganic materials, which exhibit reversible electrochromism by undergoing electrochemical reactions, are coated onto an electrode.
Solution phase electrochromics and liquid crystal configurations are limited by the diffusion speed of the electrochromic materials and supporting electrolytes towards the electrode. Thin film electrochromics, however, need only one species to diffuse towards the electrode resulting in higher switching speeds for thin film electrochromics as compared to the first two described configurations.
There remains a continuing need in the art, however, for electrochromic devices exhibiting faster switching capabilities, long-term repeatability of switching between states, reduced power consumption, and little or no color distortion.